The present invention relates to a collapsible flexible subsea pipe for transporting a fluid.
Collecting a fluid using a flexible pipe means designing a flexible pipe whose structure is such that the integrity of the bore is maintained whatever the operating conditions. The term xe2x80x9cborexe2x80x9d means the diameter of the internal element of the pipe, which is the internal carcass in the case of a rough-bore flexible pipe or the internal sealing sheath in the case of a smooth-bore flexible pipe. For flexible pipe structures of these types, reference may be made to API (American Petroleum Institute) 17B and 17J.
The design of a flexible pipe in which the integrity of the bore is maintained means that it must withstand the external pressure or hydrostatic pressure, so as to prevent it collapsing, and the internal pressure, so as to prevent it bursting, and be capable of taking up tensile and compressive loads.
Under normal operating conditions, the internal pressure is greater than the external pressure and there is therefore no risk of collapse. On the other hand, when the internal pressure decreases, for example during production shutdowns of an oil field, the pressure differential between the internal and external pressures may cause the flexible pipe to collapse.
Under such conditions, flexible pipes are designed and dimensioned so as to withstand the maximum pressure differential. As the sea depth increases, the flexible pipe is subjected to an increasingly high hydrostatic pressure, requiring the design of flexible pipes with a very high collapse resistance. On the other hand, under normal operating conditions the pressure differential between the internal pressure and the external pressure (Pint greater than Pext) decreases, generally to less than 100 bar, thereby simplifying the design of the burst resistant layers. Finally, when the internal pressure decreases, the pipe may be subjected to a very large reverse end cap effect (Pext greater than Pint), possibly causing the tensile armour plies to buckle. This reversed end cap effect may occur when laying the pipe, if it is laid empty (Pint=1 bar), or in service during a production shutdown.
The stresses developed in a flexible pipe are taken up by certain structural members of the flexible pipe, depending on whether it is of the rough-bore or smooth-bore type.
In a rough-bore flexible pipe, the external pressure is taken up by the internal carcass, which may possibly be assisted by the pressure vault so as to delay ovalization, whereas in a smooth-bore flexible pipe the external pressure is taken up by the pressure sheath and the pressure vault if an anti-collapse sheath is placed on top of the latter.
The internal pressure or the pressure differential xcex94P is taken up by the pressure vault or armour plies, the lay angle of which is about 55xc2x0 in a flexible pipe. The tensile/compressive stresses are taken up by the armour plies in both types of flexible pipe.
The design of the various structural members of the flexible pipe so as to withstand these stresses has lead inexorably to a considerable increase in the weight of the pipe, which may pose a very substantial problem in deepwater laying. The manufacturing cost is also very greatly increased.
Danish Patent Application 2000/01510 discloses a flexible pipe comprising an internal reinforcement, that takes up the external pressure and compressive forces, a pressure sheath, that provides sealing and secondarily thermal insulation, and an external reinforcement consisting of carbon-fibre armour plies wound helically with a lay angle of 55xc2x0, that takes up the internal pressure and tensile forces. The structure of the flexible pipe described in this Danish patent is one which preserves the integrity of the bore. In addition, since the compressive forces are taken up by the internal reinforcement or carcass, this means that the gaps between the turns of the carcass must disappear when it is subjected to compression. The flexible pipe will therefore shorten under the effect of compression, which may cause irreversible damage to the said flexible pipe.
The pipes disclosed in U.S. Pat. Nos. 5,176,180 and 5,908,049 are composite coiled tubings which are locally reinforced in order to increase the performance of the tubing, especially so as to increase the resistance to internal pressure with a low radius of curvature by imposing a direction of curvature, and to limit the stresses due to this curvature. It is not indicated whether the tubing can be collapsed, and then recover its initial shape.
The resistance to hydrostatic pressure in a flexible pipe is developed in API (American Petroleum Institute) 17B, section 5.4.5.1.
Pipes that can be especially wound on a reel or for other applications are disclosed in U.S. Pat. No. 3,720,235, Re 32508 and FR 1 417 987 which relate to flexible pipes that can collapse.
Patent U.S. Pat. No. 3,720,235 discloses a reinforced pipe used in diving equipment. This pipe consists of an internal tube covered with textile reinforcements that withstand collapse and that is covered with an external tube. The internal tube has fluting which allows, in the event of the tube collapsing, a space always to be provided for the fluid to flow. However, it should be noted that collapse is not due to the external pressure but to mechanical forces such as torsional or bending forces. In addition, the materials recommended in that document are xe2x80x9csoftxe2x80x9d plastics, that is those having a Shore D hardness of between 38 and 55, which have a relatively great capability of undergoing elongation. They may therefore be subjected to repeated considerable deformations without any risk of damage. The plastics known for this kind of application are generally elastomers.
The U.S. Re Pat. No. 32508 relates to a flexible pipe for refuelling ships at sea. The pipe floats and therefore is not subjected to hydrostatic pressure. However, it is stored in a reel in collapsed form and has, along the neutral fibre, longitudinal reinforcements for taking up the tensile forces. The plastic used in that application is of the rubber type (Shore D hardness of less than 38).
Patent FR 1 417 987 actually relates to a flexible pipe for transporting a pressurized fluid and comprises an internal sheath, one or more pressure-resistant reinforcements wound at 55xc2x0 and an external sheath combined with tension cables. The flexible pipe is collapsible so as to simplify its structure and the storage means. However, collapsing takes place until the facing inner walls of the internal sheath come into contact with each other over their entire surface (joining line). Such a complete collapse does not make it possible to limit the deformation. To achieve this touching collapse, it is indicated in this document that the plastic must have a relatively low Young""s modulus, like that of the rubber recommended, that is to say an elastomer. This means that if the plastic used were to be a thermoplastic, the elongations undergone by the internal sheath would cause definitive deformations that would destroy the pressure sheath.
In addition, it should be noted that the material used in the documents discussed above, and to the extent that they relate to a collapsible pipe, is generally an elastomer having a relatively low Young""s modulus as in FR 1 417 987. Given the completely collapsed shape of the pipe of FR 1 417 987, the plastic sheaths undergo locally, more particularly at the ends, very large transverse elongations of around 100 to 150%. As elastomers cannot be used in oil applications because they are not resistant to chemical attack by the fluid transported, forming blisters for example, it is preferable to use thermoplastics, whose behaviour as regards the transported fluid is better.
In offshore oil applications, the internal pressure is generally greater than the external pressure. The reverse end cap effect, and therefore the need to withstand collapse, occurs very infrequently during the life of the flexible pipe, around 50 times over 20 years of use in the case of a production line and around 200 to 300 times in the case of a gas line, the reverse end cap effect possibly occurring when laying the pipe and during production shutdowns.
The object of the present invention is to provide pipe in which the integrity of the bore is not preserved, which is collapsible and the maximum transverse elongation that the pressure sheath of which undergoes during collapse is such that there will be no local damage to the pipe, that is there will be no definitive deformation or rupture of the external fibres, even as a result of cumulative elongations. The term xe2x80x9celongationxe2x80x9d is understood to mean transverse elongation and not longitudinal elongation.
The subject of the present invention is a flexible subsea pipe for transporting a fluid, comprising at least two reinforcing armour plies wound in opposite senses about a longitudinal axis and at least one pressure sheath, which can collapse in a given direction under the action of external pressure greater than the internal pressure in the said collapsible pipe, the collapse being able to cause a transverse elongation of the said pressure sheath, characterized in that a stiffener is provided on at least one surface of the said pressure sheath, which is made of a thermoplastic, so as, during collapse, to limit the said elongation to a value of less than the elongation threshold of the said thermoplastic.
One advantage of the present invention lies in the fact that all the structural members necessary for preserving the integrity of the bore are dispensed with and the fact that it thus becomes possible to reduce the weight and the cost of the flexible pipe because collapse resistance no longer needs to be incorporated in the design.
Another advantage of the present invention is that the armour plies are designed to withstand the xcex94P (internal pressurexe2x80x94external or hydrostatic pressure) and especially to withstand the radial and axial components of the internal pressure. In addition, such a collapsible flexible pipe has a considerable advantage in the case of applications at great depth and especially against buckling of the armour plies due to the reverse end cap effect.
The reverse end cap effect causes the flexible pipe to collapse, the cross section of which in the collapsed form is smaller than the cross section of the bore in the uncollapsed form. Since the compression induced in the tensile armour plies is proportional to the cross section of the bore, it follows that the tensile armour plies are subjected to little compression as a result of the said pipe collapsing.
When the flexible pipe is of the bonded, or partially bonded, type, the entire structure will collapse under the effect of the external pressure in the event of a drop in internal pressure. The various layers of the flexible pipe will then be designed in terms of fatigue resistance according to the predictable number of collapses.
When the flexible pipe is of the unbonded type and if the reinforcing layers withstand the hydrostatic pressure, only the pressure sheath will collapse under the effect of the internal pressure or the pressure of the annulus, and it will only be necessary to design the pressure sheath in terms of fatigue resistance according to the predictable collapses.
If the reinforcing layers do not withstand the hydrostatic pressure, all the layers will be liable to collapse and therefore it will be necessary to provide means such as an impermeable plastic external sheath capable of maintaining the organization of the structure of the unbonded flexible pipe.
According to another feature of the invention, the stiffener or stiffeners maintaining the geometry of the pressure sheath during collapse cycles allows or allow the maximum elongation of the pressure sheath to be limited to a value of less than 90% of the elongation threshold. This arrangement allows thermoplastics to be used without any risk of degradation. These thermoplastics are preferably what are called semi-hard plastics, the Shore D hardness of which is greater than 55.
According to another feature of the present invention, the pipe includes two stiffeners that are arranged symmetrically in relation to the direction of collapse and that are preferably in the form of rods welded to the internal wall of the pressure sheath.